As the technology for vehicular inflatable restraints (airbags) advances, more attention is being paid to certain details of the performance of gas generators or inflators. One of these details is the shape of the pulse of gas produced by the inflator, which may be described as flowrate as a function of time.
In many of the simpler or older inflator technologies, the flowrate of gas produced either is or closely resembles a monotonically decreasing function of time as the inflation transient progresses. For example, in many of the pure pyrotechnic inflators, although there is a brief time during which ignition propagates and the inflator interior reaches its peak pressure, that time period is a very small portion of the inflation transient and by far the predominant phenomenon is that the flowrate generally decreases as the transient progresses. Other inflator technologies may involve stored gas, either as the only source of gas (in the case of pure stored gas inflators) or as a source of a portion of the gas (as is the case for hybrid inflators, which combine stored gas with solid pyrotechnics). For the discharge of stored gas, the natural shape of the discharge flowrate is approximately a decaying exponential, in which the flowrate is greatest at the beginning and continually decreases thereafter. For hybrid inflators, depending on details of the inflator design, the natural shape of the pulse may also be a flowrate which is generally decreasing as the transient progresses.
For driver side inflators, a flowrate which is generally decreasing as the transient progresses may be acceptable. However, especially for passenger side inflators, it is desirable to have a more complicated inflator performance which is referred to herein as pulse-shaping. For typical automobiles, the time (after the start of a crash) by which the passenger side inflator must be fully discharged is 50 ms to 100 ms. For such an inflation, it is desirable that the flowrate of gas out of the inflator be somewhat gentle for the first 5 to 20 ms of that period, and after that the flowrate should be relatively larger, and then toward the end of that period the flowrate should taper off. This means that when inflator testing is performed by discharging the inflator into a closed receiving tank, as is commonly done during inflator development, the pressure transient in the receiving tank should appear as a gently rising pressure vs. time trace, followed by a more steeply rising portion of the pressure vs time trace, followed by a leveling off at a final value. This characteristic is referred to as the S-shaped curve which is an example of a monotonic curve.
The S-curve is desirable principally because of the possibility of a so-called out-of-position occupant on the passenger side of a vehicle. On the driver side of a vehicle the expected position of the driver at the start of the accident is fairly well known, but on the passenger side there can be one or two children and/or adults in any of a variety of positions including relatively close to the instrument panel. If the occupant happened to be close to the instrument panel at the beginning of bag deployment, when the fill rate of a non-pulse-shaped inflator is most rapid, there would be the possibility of a bag-induced injury. The gentle early flowrate of gas from the inflator is helpful so that the airbag can perhaps reposition or cushion an out-of-position occupant during the early portion of the inflation without subjecting him or her to harmful decelerations. The more rapid flowrate later is necessary so that the airbag completes its inflation within the time period of the typical crash. Finally, the tapering off at the end of pulse is a natural consequence of the inflator nearing the end of its discharge process. Having a brief gentle early period during the inflation can help to lessen the forces on the bag and associated anchoring structures as the bag begins to unfold. If the flowrate were excessive in the very early portions of the transient, such forces could tear the bag.
There are some techniques that have been used or are presently being used to produce pulse-shaping. As mentioned, pure pyrotechnic inflators have a slight natural tendency to produce an S-shaped curve, but the portion of the curve which exhibits the gentle build-up tends to be only of a very brief duration of the order of a few milliseconds, not as much as may be desired for pulse-shaping at most. This is described in Society of Automotive Engineers paper 920120, Advances in Analytical Modeling of Airbag Inflators, by Peter Materna. Some pyrotechnic inflators are also designed with the pyrotechnic subdivided into more than one chamber in order to ignite the pyrotechnic in stages so as produce pulse-shaping. Other types of inflators produce pulse-shaping by some means separate from the combustion process, means which essentially vary the exit area through which gas can flow. For example, some inflators include a movable object in the exit path such that as the object moves under the influence of a pressure difference it uncovers additional exit area. Because of the very large internal pressures at which inflators are typically designed to operate, the inertia alone of a reasonably sized movable object is not sufficient to produce the desired duration of pulse-shaping. Thus, the movable object is backed by an energy absorbing substance or component, such as a crushable rubber-like substance or a crushable metal honeycomb structure. However, in such inflators, the movable part involves close-fitting parts such as pistons and cylinders which may have risk of binding or sticking, particularly given the large unpredictable accelerations found in vehicles during crashes. These inflators also do not deal with the question of how to provide this pulse-shaping over a wide range of inflator initial temperatures. The inflator initial temperature influences not only the characteristics of the energy absorbing material (especially in the case of rubber) but also how the pressure in the interior of the inflator (which acts on the energy-absorbing material) may change with ambient temperature. This latter influence is especially present if stored gas is involved. There are also some pulse-shaping techniques which involve two actuating events, one to cause the gentle fill portion and another to cause the rapid fill portion of the inflation. Typically, an electronic timing circuit sequences the two events. This could involve, for example, igniting two different pyrotechnic charges sequentially. However, the drawback of such a system is that from a reliability point of view, there is more opportunity for the system to fail to operate correctly. From a reliability point of view, it would be preferable if there were only a single actuating event such as ignition of a pyrotechnic, and all other events including pulse-shaping followed as a consequence of that one actuating event.
Overall, it can be said that there still is not a completely satisfactory method for producing pulse-shaping at the conditions of extremely high pressures and short time scales and widely varying initial temperatures typically found in airbag inflators.